In the art of drilling wells the drilling fluid must act as a liquid medium of controlled viscosity for removing cuttings from the bore hole. It should also prevent excessive amounts of fluid from flowing from the bore hole into surrounding formations by depositing on the wall of the hole a thin but substantially impervious filter cake. It desirably possesses a gel structure of sufficient strength to hold solids in suspension, particularly during any time the fluid is not circulating.
The nature of the formation penetrated can change drilling fluid properties. Certain formations, such as gypsum, will effectively "cut" the drilling mud so that the clay particles are flocculated and the viscosity becomes too high. A heaving shale formation absorbs water from the drilling mud and/or swells choking off the circulation of drilling mud. Gelatin and/or thickening of the drilling fluid due to the higher temperatures is sometimes a problem in deeper wells. This tendency to gel or thicken is most pronounced in muds containing water-swelling clays such as bentonite.